an idea, behavior, style, or usage that spreads from person to person within a culture

The dictionary also give the historical background of the word:

In his 1976 book The Selfish Gene, British scientist Richard Dawkins defended his newly coined word meme, which he defined as “a unit of cultural transmission.” Having first considered, then rejected, “mimeme,” he wrote: “‘Mimeme’ comes from a suitable Greek root, but I want a monosyllable that sounds a bit like ‘gene.’ I hope my classicist friends will forgive me if I abbreviate ‘mimeme’ to ‘meme.’” (The suitable Greek root was mim-, meaning “mime” or “mimic.” The English suffix -eme indicates a distinctive unit of language structure, as in “grapheme,” “lexeme,” and “phoneme.”) “Meme” itself, like any good meme, caught on fairly quickly, spreading from person to person as it established itself in the language.

So, the word meme is not just about funny or satire picture (with words on it) that quickly spread around the web (“going viral”). It’s about the spreading of certain issue.

Here is the example of meme:

It tells us that every paper trumpet that sold by Indonesian street vendors has already been tested by its seller (and also the creator).